Video Streaming for Mobile App

I'm building an iOS app for a client that allows users to pay a subscription and unlock additional content within the app. Part of the additional content will be videos which need to be streamed from a server... but I'm not sure whether we should use a hosting service (like Amazon CloudFront or Wowza, perhaps?) or roll our own solution.

Have any of you had experience with either of these options? I looks like this is supported natively by nginx, which we're currently using as our rProxy, but I'd like to hear some thoughts about that. I would be somewhat concerned about saturating our server's 1Gb network connection too...

Whatever the solution, we must be able to verify a person's account before they can access the video content. Variable bitrate is also desirable, and the ability to support >500 concurrent users. This company is also a new startup, so subscription costs are an important factor.